The present invention relates to under-roofing materials and has specific reference to a material of this kind intended for covering the roof structure and receiving the ultimate or topmost roofing material, and consisting of elements assembled in overlapping relationship, which are impervious to water, snow or ice, and pervious to air or vapor.
It is known to those conversant with the art that roofs covered with roofing materials made of discrete elements must be isolated and protected against the penetration of water and, snow.
These discrete elements may consist of tiles, slates, or any other reliable weatherproof materials, their imperviousness resulting from their mutual overlapping in the fashion of fish scales.
As a rule, this under-roofing material is disposed between the tile support means and the framework.
Hitherto known under-roofing materials are perfectly tight, notably in the case of plastic films or tarred felt sheets.
However, plastics material and other materials such as impregnated cellulose sheets have neither the porosity required to enable the roof to "breath" while preserving a satisfactory water-tightness, nor the strength necessary to permit their use in roof construction without any danger for the roofer or tiler.
On the other hand, for reasons of economy and low-weight requirements, these materials have moderate mechanical properties involving a certain handling risk for the roofer.
This double problem will be better understood from the following two illustrative examples:
1. One of the materials utilized is a polyethylene film. This perfectly fluid-tight film, though preventing any water from penetrating into the building, will retain this water between the framework and the tile support means. Consequently, since the roof cannot "breath" the tile supports are liable to rot after a relatively short time, unless the user perforates the polyethylene film at many places for restoring the air circulation, but in this case the protection against the penetration of water is lost.
2. When used by the roofer this material is attended by certain handling dangers. In fact, when fitting the battens, the roofer cannot see the frame, so that he cannot locate with precision the bearing points on which he can walk, and therefore he runs the risk of falling through the roof.
The French Pat. No. 2,098,475 discloses a roofing insulation strip consisting of thermoplastic filaments or fibres heated to provide the necessary water-tightness while allowing the passage of air therethrough.
The U.S. Pat. No. 3,756,895 also describes a roof covering consisting of several layers of glass fibres associated with a microporous surface layer or membrane consisting of polyester and polyurethane fibres.